The present invention relates to Gifford McMahon (GM) type pulse tube refrigerators. Coldheads of such cryogenic refrigerators include a valve mechanism, which commonly consists of a rotary valve disc and a valve seat. There are discrete ports, which, by periodic alignment of the different ports, allow the passage of a working fluid, supplied by a compressor, to and from the regenerators and working volumes of the coldhead.
GM type refrigerators use compressors that supply gas at a nearly constant high pressure and receive gas at a nearly constant low pressure. The gas is supplied to a reciprocating expander that runs at a low speed relative to the compressor by virtue of a valve mechanism that alternately lets gas in and out of the expander. Gifford, U.S. Pat. No. 3,205,668, discloses a multi-ported rotary disc valve that uses the high to low pressure difference to maintain a tight seal across the face of the valve. This type of valve has been widely used in different types of GM refrigerators as shown for example in Longsworth, U.S. Pat. No. 3,620,029, and Chellis, U.S. Pat. No. 3,625,015. This type of valve has the disadvantage of requiring an increased amount of torque as the diameter is increased to accommodate larger ports or ports for multiple valves.
A Pulse Tube refrigerator was first described by W. E. Gifford in U.S. Pat. No.3,237,421, which shows a pulse tube, connected to valves like the earlier GM refrigerators. It also shows a pulse tube expander connected directly to a compressor so it pulses at the same speed as the compressor. This is equivalent to a Stirling cycle refrigerator.
Early pulse tube refrigerators were not efficient enough to compete with GM type refrigerators. A significant improvement was reported by Mikulin et al. in 1984, (E. I. Mikulin, A. A. Tarasow and M. P. Shkrebyonock, ‘Low temperature expansion (orifice type) pulse tube’, Advances in Cryogenic Engineering, Vol. 29, 1984, p.629) and a lot of interest ensued in looking for further improvements. Descriptions of major improvements since 1984 can be found in S. Zhu and P. Wu, ‘Double inlet pulse tube refrigerators: an important improvement’, Cryogenics, vol.30, 1990, p.514; Y. Matsubara, J. L. Gao, K. Tanida, Y. hiresaki and M. Kaneko, ‘An experimental and analytical investigation of 4K (four valve) pulse tube refrigerator’, Proc. 7th Intl Cryocooler Conf., Air Force Report PL-(P-93-101) ,1993, p166-186; S. W. Zhu, Y. Kakami, K. Fujioka and Y. Matsubara, ‘Active-buffer pulse tube refrigerator’, Proceedings of the 16th Cryogenic Engineering Conference, 1997, p. 291-294; and J. Yuan and J. M. Pfotenhauer, ‘A single stage five valve pulse tube refrigerator reaching 32K’, Advances in Cryogenic Engineering, Vol. 43, 1998, p.1983-1989. Additional disclosure of improvements can be found in Lobb, U.S. Pat. No. 4,987,743.
All of these pulse tubes can run as GM type expanders that use valves to cycle gas in and out of the pulse tube, but only the single and double orifice pulse tubes have been run as Stirling type expanders. Stirling type pulse tubes are small because they operate at relatively high speed. The high speed makes it difficult to get to low temperatures so GM type pulse tubes running at low speed are typically used for applications below about 20 K. It has been found that best performance at 4 K has been obtained with the pulse tube shown in FIG. 9 of Gao, U.S. Pat. No. 6,256,998. This design has two valves controlling flow to the regenerator, and four valves controlling flow to the warm ends of the pulse tubes, which open and close in the sequence shown in FIG. 11 of U.S. Pat. No. 6,256,998. The single stage version of this pulse tube has four valves, two to the regenerator and two to the pulse tube, thus this control is commonly referred to as four-valve control. These valve functions are commonly implemented by the use of a multi-ported rotary disc valve.
When designing a valve that has a disc rotating on a stationary seat it is customary to have one or more ports in the seat that connect to the regenerator, with gas flowing to and from the regenerator through the same ports. While most GM refrigerators use two ports and have two cooling cycles per revolution of the valve disc, three ports have been used, as described in Longsworth, U.S. Pat. No.4,430,863. A single port valve that provides one cycle of cooling per revolution for a GM expander is described in Asami, et al., U.S. Pat. No. 5,361,588. This valve is different from conventional rotary valves in having the high-pressure gas from the compressor act against the valve seat to push it into the face of a rotary valve. A bearing holds the valve disc against the axial force of the valve seat, rather than transferring it as an axial load to the motor shaft. The flow of gas in this arrangement is reversed from the conventional arrangement shown in previous patents. High-pressure gas flows into the center port and low-pressure gas is discharged to the outer perimeter of the valve.
FIG. 11 of U.S. Pat. No. 6,256,998 shows different timing for gas flowing to and from the 2nd stage pulse tube, PT2, relative to the 1st stage pulse tube, PT1, but it doesn't show another important characteristic of these valves, namely that the size of the orifice in each valve is different. It is necessary to control the amount of gas that flows to each pulse tube and also to have the same amount of gas return to low pressure as flowed in from high pressure. Because the densities are different the orifice sizes in the valve for each pulse tube have to be different.
In a rotary face valve, the ports in the valve seat to the regenerator are on the same diameter circle, or track, because both the high-pressure supply and low-pressure return are connected alternately by the slots in the rotating disc. For a valve disc that has a single cooling cycle per revolution it is necessary to have each of the four ports to the pulse tubes be-on ports-at-different radii, with sufficient radial separation so there is no leakage from one to another. The valve thus has five tracks, one for the flow to and from the regenerator, and four for the flow to and from the pulse tubes. This increases the diameter of the valve and consequently significantly increases the torque.
It is an object of the present invention to reduce the diameter of, and the torque required to turn, a rotary face valve for use in a multi-valve pulse tube.